Double-current audio-frequency telegraphic transmission system



Oct. 25, 1949. R. P. LEROY 2,486,041-

DOUBLE-CURRENT AUDIO-FREQUENCY. TELEGRAPHIC I TRANSMISSION SYSTEM FiledNov.- 6, 1945 f s? I F BAND PASS V, AMPLIFIER FILTER Xm i I I i iAUXILIARY I :DM MODULATOR I oEnoo IIAToR I I I I I i I I l I L -J @ImzowIIAIIII AMPLIFIER ms FILTER L03 LOCAL RECEIVER F I9. I.

(PRIOR ART) I U 3 J BAND PASS AMPLIFIER FILTER BL OMBINEDRRECTIFIEREBLOCKER NARROW BAND AMPLIFIER .PASS FILTER I.oc

LOCAL RECEIVER Fl 9. a .zwvavrae Patented Oct. 25, 1949 DOUBLE-CURRENTAUDIO-FREQUENCY TELEGRAPHIC TRANSMISSION SYSTEM Robert Pierre Leroy,Paris, France, assignor to Compagnie Generale DElectricite, Paris,

France, a corporation of France Application November 6, 1945, Serial No.627,025 In France March 12, 1943 Section 1, Public Law 690, August 8,1946 Patent expires March 12, 1963 3 Claims.

The present invention relates to double current telegraph transmissioncircuits using a code of two elements or trains, of which the firstconsists of a transmission of signals of a carrier of audiofrequency andthe second of audio frequency signals of the same frequency but 180 outof phase. It is well known that, as described in French Patent No.803,109, that in such systems which are called audio-frequency telegraphtransmission systems with phase inversion, the telegraph signals arereconstituted at the receiver by demodulation of the received wave bymeans of a reconstituted carrier wave used for demodulating. Thereconstituted carrier wave is necessary because without it, one cannotat the receiving station arrange to provide a phase for comparison topermit of interpreting the signals received.

According to whether the phase of the reconstituted carrier Wave iscorrect or not, so the succession of reconstituted signals obtained willbe a faithful reproduction or the inverse of that of the signalstransmitted.

To ensure agreement, it is necessary that the carrier wave serving fordemodulation is initially started off with a phase or sense or polaritypredetermined with relation to the first received wave, which determinesthe direction or sense or polarity of the first reconstituted telegraphcurrent impulse obtained, which direction can be chosen beforehand to beidentical with that which will be suitable for the first impulsetransmission.

While thus ensuring at the end of the transmission the identity of thesignals transmitted and obtainedthe first transmission may be chosen asa neutral signalthe correct reception of any succession of signalswhatever is assured provided the circuit is not broken. To satisfy thiscondition, the present invention has for its object a method of polingthe initial phase of the carrier wave reconstituted at the receiver andis characterised in that in order to obtain demodulation, the incidentwave is combined with an auxiliary wave during the reception of thefirst train of waves.

In such a double current system of telegraph transmission, the sendingstation comprises a modulator at the control terminals of which thetelegraph code transmitter applies a control current which is at certaininstants a rest current of one polarity, and at other instants a markingor operating, or work current of opposite polarity. The inversions inpolarity of'this control current has for its effect to invert the phaseof the telegraph current sent in the line, every change of polarity ofthe control current causing an inversion of phase of the telegraph linecurrent.

To each character transmitted, there corresponds, therefore, theapplication to the modulator of a succession of pulses of alternatingcurrent which are alternately rest currents and work currents, accordingto a determined code, and, hence, correspond to the transmission on theline of a succession of currents of phases which are alternatelyinverted.

For example, in the International start-stop code, to the letter S therecorresponds, after the sending of a work or marking current pulsecorresponding to the start, the transmission, during an elementaryinterval, of a rest current, then, during the following interval, of awork current, then during a following interval of a rest current, thenduring two following successive intervals of a work current, and,thereafter, finally of a rest current corresponding to stop. To each oneof these inversions there corresponds a change of the phase of thecurrent which was sent on the line, there being five changes of phase inthe course of the transmission of the letter S.

At the receiving station, there is applied to a demodulator, on the onehand, the received current received at the end of the line, includingits inversions of phase, and, on the other hand, the reconstitutedcarrier frequency current as it has been set up in the line at thebeginning of the transmission, but maintaining its initial phase withoutany change. Thus, the current delivered from the output terminals of thereceiving demodulator will change in sense or direction at the same timethat the telegraph current in the line will change its phase.

Hence, it can be said that every change of sense or direction of thecurrent taken oil at the output terminals of the demodulator of thereceiv ing station corresponds to a change of sense or direction of thecurrent applied to the control terminals of the demodulator of thetransmitting station. But, on first consideration, it cannot be knownwhether to a given received pulse of rest current, there corresponds, atthe transmitting station, a pulse of rest current or a pulse of Workcurrent.

In order to assure agreement between the senses or directions of theoriginal currents of reception and those of transmission, provision ismade so that the first current pulse taken ofi at the output of thedemodulator of the receiving station can be of only one determined senseor direction, and it will always be provided that the transmission willbe commenced by the sending of a current of this same determined senseor direction, for example, a rest current pulse, if it has been arrangedthat the demodulator of the receiving station shall, on its first pulse,deliver initially a rest current.

The present invention has for an object to provide means forunambiguously determining at the receiving station, the phase of thecurrent at the output of the demodulator when this receiving stationreceives a first signal pulse after an interruption of the current inthe line has occurred, due to any circumstances.

The present invention is applicable to a known type of system whereinthere is used for the re constituting of the carrier current, anauxiliary modulator energized, on the one hand, by the signals receivedat the end of the telegraph line, and, on the other hand, by the signalsdelivered at the output of the principal demodulator. Such a system hasbeen described in British Patent 449,999, and the corresponding U. S.Patent 2,108,117 of Gardere and Pages.

In order to assure with such a system the establishment at the output ofthe auxiliary modulator of the re-constituted carrier current, it isnecessary that, in the absence of current delivered at the output of theprincipal demodulator, which current is applied to the control terminalsof the auxiliary modulator, that this latter auxiliary modulator shouldfurnish at its output terminals a current of phase which is definitelydetermined with relation to the current at its input terminal.

In the mentioned system of the Gardere and Pages patent, this result isobtained by causing a dissymmetry in the auxiliary modulator by a choiceof uni-directional conducting elements of difierent characteristics.There arises herefrom, however, a variation of the reconstituted carriercurrent according to the direction of the current applied to the controlterminals, that is to say, according to the sense or polarity of thesignal pulses. The present invention provides a device which permitsattaining these same results without it being necessary to introducedissymmetry into the auxiliary modulator.

The present invention is characterized by the fact that it comprises,for the supplying of the control circuit of the principal demodulator atthe instant of the arrival at the end of the line of the first signalcurrent pulse, a derived circuit taken off the said line, and by whichthe received cur rent is fed to this control circuit through the meansof a blocker whose blocking control voltage is taken through a low passfilter from the output terminals of a rectifier supplied by thereconstituted carrier voltage.

The present invention will be better understood by referring to theaccompanying drawing which illustrates circuits by which the methodaccording to the present invention can be realised.

Fig. 1 is a standard circuit arrangement in audio-frequency telegraphyusing phase inversion as used in the prior art.

Fig. 2 illustrates the same circuit arrangement modified in accordancewith the present invention.

In the conventional arrangement of the prior art shown in Fig. 1, thesignals coming over the line L toward the receiving station, traverse inaddition to the bandpass filter F1, the amplifiers V1 and V2, thedemodulator DM, the auxiliary modulator Xm and the narrow bandpassfilter F2.

To the output of demodulator DM the local receiving circuit LOC isconnected, which circuit consists of a telegraph receiver or anintermediate relay. The demodulator DM uses for demodulation thereconstituted carrier frequency output of auxiliary modulator Xm whichpermits the obtaining of the original direct current double currentdouble current d. c. signals. These are the signals which by modulatingthe incident wave derived from the line and incident on auxiliarymodulator Xm reconstitute the carrier at the output of auxiliarymodulator Xm, the narrow band-pass filter F2 of relatively long timeconstant serving to preserve the phase for a sufficient time to permitthe telegraph current to become inverted.

If in the prior art arrangement of Fig. 1 the auxiliary modulator Xm wasperfectly symmetrical, reception could not be obtained. It is thereforenecessary to give said auxiliary modulator Xm a definite bias duringnon-operating conditions.

According to the present invention one applies to the input of amplifierV2 without passing 0 through narrow band filter F2, in addition to thevoltage derived from the output of the bandpass filter F1, the voltagederived from the incident signals coming in on the line and passingthrough a blocking device BL having a slight attenuation duringnon-operating conditions but which during actual signal transmission canbe blocked as soon as the normal energization of amplifier V2 by anauxiliary modulator is assured, either by the telegraph currentcorrected and filtered by a low-pass filter or by derivation from thedemodulated carrier wave taken from the output of amplifier V2.

Such blocking is able to bring about in known blocking systems anattenuation of 5 nepers, and the lack of symmetry applied to the inputof amplifier V2 according to the laws of blocking is without importance.A blocker may, for the present purposes, be considered to be aquadripole having attenuation and cut-01f characteristics controlled byan applied controlled current which can be realized with dry rectifiersarranged in known circuit arrangements.

It will be borne in mind that what one calls a blocking device is abridge network or quadripole having its attenuation regulated by acontrol current which can be obtained by means of dry rectifiersarranged in accordance with known circuit arrangements.

Fig. 2 shows the arrangement of Fig. 1 modified according to thepreceding description. The con trol of the blocking action of blockingdevice BL is effected by means of the carrier wave derived from theoutput of amplifier V and rectified by means of a device Rd which is acombined rectifier and filter. The advantage of this arrangement is thatone can obtain a substantially constant blocking current by a simplefilter.

Auxiliary modulator Xm may consist of a rectifier bridge as may alsodemodulator DM. Blocker BL may consist of a rectifier quadripole.Combined rectifier and filter Rd may consist of a rectifier unitquadripole with a condenser connected across to opposite corners. Theamplifier V2 may have its input transformer coupled by a transformerhaving two primaries respectively connected to the outputs of blocker BLand filter F2.

The initial signal passes directly from the line through blocker BL tothe input of amplifier V2 without passing through filter F2, since thereis at that instant no output of amplifierv to be rectified by rectifierRd and then applied to the control terminals of blocker BL to causeblocking action. An'instant later the portion of the signal derived fromthe line and traversing auxiliary modulator Xm and filter F2 hasreachedamplifier V2,.whose rectified output of the first signal pulse throughthe blocker and subsequent pulses through the filter are delayedsomewhat in passage to rectifier Rd, is then applied to the corn trolterminals of blocker BL to block it. By the time the initial signalelement has had time to pass through the unactuated blocker and theamplifier V2 and the rectifier Rd, to the control terminals of blockerBL to block it, the next signal element through demodulator DM andfilter F2 has had time to establish itself, and the path through blockerBL is no longer needed. In this way, the initial signal element passingthrough the blocker BL controls the first signal pulse delivered.

The blocking device BL is connected between the output of band passfilter F1 and the input of modulator V2. The initiation of this controlcurrent from amplifier V2 through device Rd is made sufliciently slowand smooth so that at the moment when it causes suppression of thesignal at the output of blocking device BL, the normal energization ofamplifier V2 by auxiliary modulator Xm through narrow band pass filterF2 has been able to establish itself.

If on the other hand it was desired to use the telegraph current takenfrom the output of DM and rectified, it would be necessary to compensatefor the variations in the transmission from one signal pulse to anothersignal which it is not possible to attain by means of an inertialessblocking device; a filter having a time constant lying between one-halfand the full value of the time constant of the audio-frequency filtersgives a reduction of about one half of the rectified current in thetransmission from one signal pulse to another. It is necessary that thisreduction still permits of a very slightly attenuated blocking in orderthat the blocking device BL may not be the cause of certain difficultiesarising.

Care is taken to provide a suitable condition of the impedances in thearrangement of Fig. 2, so that under the operating conditions, when theblocker BL is blocked, this blocker delivers practically no energy tothe input of amplifier V2 and. so that when unactuated and not blocking,when its attenuation is small, the blocker BL does not constitute animpedance which is so small as to involve the risk of draining off toolarge a fraction of the energy available at the input of amplifier V2.Suitable values of impedance are easy to realize through familiarprocedures of current technique.

The rectifier Rd should represent an impedance of sufliciently greatmagnitude to not drain off too large a fraction of the energy availableat the output of amplifier V2, and of such value as to furnish, when there-constituted carrier wave at the output of amplifier V2 has its normalamplitude, a rectified voltage which is of sufficient magnitude toassure the blocking of blocker BL.

The blocking device. BL is of a known type and is designed to haveslight attenuation in the absence of the application of control currentfrom the output of amplifier V2 through device Rd. When the controlcurrent is applied to the blocking device BL, however, its attenuationcan attain a value as high as five nepers. The dissymmetry'resultingtherefrom is not important under normal operating conditions.

The output of the narrow band pass filter F2 feeds through a transformerT, the input of. the amplifier V2. This transformer T comprises twoprimary windings and a secondary winding, this latter feeding the inputof the amplifier V while one of the primary windings is energized fromthe output of narrow band pass filter F2. The other primary winding isconnected to the output of blocking device BL. It is well understoodthat care has to be taken to obtain suitable impedance characteristicsin order that in the position where it is blocked, the blocking deviceBL shall drain practically no energy at all from the input of theamplifier V2 and in order that in its nonoperative condition, where theattenuation is small, the blocking deviceBL does not constitute too lowan impedance with the risk of reducing by too great a fraction theenergy available at the input of amplifier V2. Suitable impedancecharacteristics can be easily provided by methods well-known in thepresent state of the art.

So, the rectifier Rd should represent a sufficient impedance in ordernot to reduceby an excessive fraction the energyuavailable at the outputof amplifier V2 and to furnish when the reconstituted carrier at theoutput of amplifier V2 has its normal amplitude, a rectified voltagesufficient to ensure the blocking of blocking device BL.

In arrangements such as that described, the output modulator is usuallyarranged in a circuit at the output of the amplifier V2 and is suppliedwith a reconstituted carrier Wave (it can besides be, and it isgenerally the case. that. one of the communicating stations is suppliedwith energy from a local generator); in the absence of control current,this modulator ought to have a definite condition.

Immediately after receiving the beginning of the first signal, thesignal supplied by the auxiliary modulator Xm is delayed, as has beenshown, by the filter F2. The control circuit of the blocking device BLis, therefore, not energized; this is then open and allows the incidentsignal to pass on to the control circuit of demodulator DM. DemodulatorDM thus receives the said incident signal at the same time at both itsinput and control circuits; the conditions of agreement of phase are,therefore, automatically realized and demodulator DM yields a signal ofcorrect phase at its output.

Auxiliary modulator Xm is then supplied, in its turn, at its controlcircuit with the correct signal from demodulator DM; it, therefore,delivers at its output terminals, a signal in phase with the signal atits input terminals. When the retarding effect of the filter F is nolonger felt, the output auxiliary signal of modulator Xm is delivered tothe control circuit of demodulator DM instead of the signal throughblocker BI, which is blocked since its control circuit is now energized.

It is obviously necessary that blocker BL should not be blocked beforethe signal from auxiliary modulator Xm has had time to reach demodulatorDM. This is Why the arrangement including rectifier Rd comprises, inaddition to a rectifier, a filter having a time constant longer thanthat of F2. So, as long as the filter F2 blocks the signal fromauxiliary modulator Xm, the control circuit of blocker BL is notenergized and blocker BL allows the incident signals to pass.

Afterwards, blocker BL does not play any part, being permanently blockedin normal operation by the re-established carrier wave from auxiliarymodulator Xm which energizes its control circuit; blocker BL, therefore,only 'serves at the moment of commencing to receive the first signal.

The control circuit of blocker BL could equally be supplied by thetelegraph current taken from the output of demodulator DM and rectified,but it would then be necessary to compensate the variations in thiscurrent which are produced in passing from one signal to another andwhich would influence the operation of the blocking device; by reason ofits absence of inertia; this compensation would entail complicatedarrangements.

Care must be taken to achieve suitable impedance conditions such thatwhen the blocking device BL is closed it takes substantially no energyfrom the input of the amplifier V2 and when it is open it takes only areduced fraction of the available energy at the input of this amplifier.

Also, the rectifier Rd should have sufiicient impedance not to take anexcessive fraction of the energy available at the input of V and supply,when the output current of V2 is of normal ampli tude, rectifiedpotential suflicient to insure the blocking of BL.

I claim:

1. In low frequency carrier telegraph receiving apparatus, an incomingline, a demodulator having principal input terminals and controlterminals and adapted to receive modulated alternating current signalsand deliver corresponding direct current signals, a telegraph receivingdevice connected to the output of said demodulator,

a principal band pass filter adapted to pass a restricted band of signalmodulated carrier frequencies and having its input connected to saidline, a principal amplifier connected between the output of saidprincipal band pass filter and the input of said demodulator, anauxiliary modulator having principal input terminals connected to theoutput of said principal band pass filter and further having controlterminals, a direct connection between the control terminals of saidauxiliary modulator and the direct current output of said demodulator,an auxiliary narrow band pass filter having its input connected to theoutput of said auxiliary modulator, an auxiliary amplifier having itsinput connected to the output of said auxiliary filter and having itsoutput connected to control terminals of said demodulator, a blockingunit having an input connection and a control connection and an outputconnection, said input connection being connected to the output of saidprincipal filter, said blocking unit being adapted in the absence ofenergization of its control connection to pass signals with negligibleattenuation, and when its control connection is energized tosubstantially attenuate signals, and a rectifier connected between theoutput of said auxiliary amplifier and said control connection forapplying blocking control energy to the control connection of saidblocking unit when substantial energy is delivered by said auxiliaryamplifier, and a second connection from the input of said auxiliaryamplifier to the output of said blocking unit, whereby the identicalfrequencies of the alternating currents applied at the input 8 terminalsand control terminals of said demodulator produce a signal varyingcurrent delivered from its output terminals, and additional signal inputto the input of said auxiliary amplifier and to the control terminals ofsaid demodulator is applied when said blocking unit is not blocked, andthe phase of the first impulse of the reconstituting carrier frequencyapplied at the receiver is identical with the phase of the first signalimpulse received.

2. Apparatus as recited in claim 1, and a trans former interposed in theinput connections of said auxiliary amplifier, said transformer havingtwo primary windings and one secondary winding, said secondary windingbeing connected to the input of said auxiliary amplifier, one of saidprimary windings being connected to said auxiliary filter and the otherof said primary windings being connected to the output of said blockingunit.

3. In low-frequency carrier telegraph receiving apparatus, an incomingline, a channel separating filter having its input connected to saidline, a demodulator having input connections and control connections andoutput connections, a principal amplifier having its input connected tothe output of said channel separating filter and having its outputconnected to the input connections of said demodulator, a telegraphreceiving device connected to the output connections of saiddemodulator, an auxiliary modulator having input terminals and controlterminals and output terminals, the input terminals of said auxiliarymodulator being connected to the output of said channel separatingfilter, the direct current output connections of said demodulator beingconnected to the control terminals of said auxiliary modulator, anauxiliary band pass filter having its input connected to the outputterminals of said auxiliary modulator, an auxiliary amplifier having itsinput terminals connected to the output of said auxiliary filter and itsoutput terminals connected to the control connections of saiddemodulator, a blocking unit having input terminals and output terminalsand control terminals and adapted to pass signals with negligibleattenuation when its control terminals are not energized and tosubstantially attenuate signals when its control terminals areenergized, rectifier means having its alternating current inputconnected to the output of said auxiliary amplifier and its outputconnected to the control terminals of said blocking unit, the inputterminals of said blocking unit being connected to the output of saidchannel separating filter, and coupling means adapted and connected fordelivering the output from the output terminals of said blocking unit tothe input terminals of said auxiliary amplifier.

ROBERT PIERRE LEROY.

REFERENCES GITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,559,642 Nyquist Nov. 3, 19251,707,814 Pernot Apr. 2, 1929 2,272,840 Hammond Feb. 10, 1942 2,409,577Matson Oct. 15, 1946

